Closest Point Of Approach Research Articles

The effect of Doppler rate on small aperture bearing estimation

Passive estimation of the angle of arrival using a small aperture array outdoors can be adversely effected by high rates of frequency chirping. This is caused by either rapid change in source frequency and/or rapid change in Doppler frequency due to the motion of the source. Consider two microphone signals simultaneously sampled and analyzed for phase difference via FFT. The time difference of arrival across the small aperture array is small compared to the FFT buffer size, so there are slightly different starting and stopping frequencies for each microphone during the FFT input buffer record. However, the chirp (rate of change in Doppler or source frequency) leads to a significant phase error between the two microphones. An upwards chirp moves the estimated bearing erroneously towards 0 degrees while a downward chirp moves the estimated toward 180 degrees. For tracking aircraft from the ground it is not unusual to experience high rates of change in Doppler frequency near the closest point of approach. This paper shows the extent of the error from practical Doppler and source chirps and how to correct for the bearing error.

Novel precision enhancement algorithm with reduced image noise in cosmic muon tomography applications

In this paper, we present a new algorithm that improves muon-based generated tomography images with increased precision and reduced image noise applicable to the detection of nuclear materials. Cosmic muon tomography is an interrogation-based imaging technique that, over the last decade, has been frequently employed for the detection of high-Z materials. This technique exploits a magnitude of cosmic muon scattering angles in order to construct an image. The scattering angles of the muons striking the geometry of interest are non-uniform, as cosmic muons vary in energy. The randomness of the scattering angles leads to significant noise in the muon tomography image. GEANT4 is used to numerically create data on the momenta and positions of scattered muons in a predefined geometry that includes high-Z materials. The numerically generated information is then processed with the point of closest approach reconstruction method to construct a muon tomography image; statistical filters are then developed to refine the point of closest approach reconstructed images. The filtered images exhibit reduced noise and enhanced precision when attempting to identify the presence of high-Z materials. The average precision from the point of closest approach reconstruction method is 13 %; for the integrated method, 88 %. The filtered image, therefore, results in a seven-fold improvement in precision compared to the point of closest approach reconstructed image.

First Calibration Results for the SARAL/AltiKa Altimetric Mission Using the Gavdos Permanent Facilities

This work presents the first calibration results for the SARAL/AltiKa altimetric mission using the Gavdos permanent calibration facilities. The results cover one year of altimetric observations from April 2013 to March 2014 and include 11 calibration values for the altimeter bias. The reference ascending orbit No. 571 of SARAL/AltiKa has been used for this altimeter assessment. This satellite pass is coming from south and nears Gavdos, where it finally passes through its west coastal tip, only 6 km off the main calibration location. The selected calibration regions in the south sea of Gavdos range from about 8 km to 20 km south off the point of closest approach. Several reference surfaces have been chosen for this altimeter evaluation based on gravimetric, but detailed regional geoid, as well as combination of it with other altimetric models.Based on these observations and the gravimetric geoid model, the altimeter bias for the SARAL/AltiKa is determined as mean value of −46mm ±10mm, and a median of −42 mm ±10 mm, using GDR-T data at 40 Hz rate. A preliminary cross-over analysis of the sea surface heights at a location south of Gavdos showed that SARAL/AltiKa measure less than Jason-2 by 4.6 cm. These bias values are consistent with those provided by Corsica, Harvest, and Karavatti Cal/Val sites. The wet troposphere and the ionosphere delay values of satellite altimetric measurements are also compared against in-situ observations (−5 mm difference in wet troposphere and almost the same for the ionosphere) determined by a local array of permanent GNSS receivers, and meteorological sensors.

Close approach analysis of space objects and estimation of satellite-debris collision probability

Purpose – The purpose of this paper is to perform a comparative study of two propagation models and a prediction of proximity distances among the space objects based on the two-line element set (TLEs) data, which identifies potentially risky approaches and is used to compute the probability of collision among the spacecrafts. Design/methodology/approach – At first, the proximities are estimated for the mentioned satellites using a precise propagation model and based on a one-month simulation. Then, a study is performed to determine the probability of collision between two satellites using a formulation which takes into account the object sizes, covariance data and the relative distance at the point of closest approach. Simplifying assumptions such as a linear relative motion and normally distributed position uncertainties at the predicted closest approach time are applied in estimation. Findings – For the case of Iridium-Cosmos collision and the prediction of a closest approach using available TLE orbital...

Parameter estimation of underwater moving sources by using matched Wigner transform

According to the acoustical Doppler effect, an improved narrow-band technique is proposed to estimate underwater source’s motion parameters including speed, closest distance, rest frequency, and closest point of approach (CPA) time, by using a single receiving sensor. First, the matched Wigner transform (MWT), which produces a time–frequency distribution (TFD) with optimal energy concentration and without inner artifacts for a high-order polynomial phase signal, is defined. On the basis of the MWT, a method is developed for the accurate instantaneous frequency (IF) estimation of Doppler signal. Moreover, we deduce that the Wigner–Ville distribution of the Doppler signal has a peak at the center of the TFD. Thereafter, the CPA time is directly estimated. When the CPA time is obtained, the other motion parameters can be easily estimated by using a simplified amplitude weighted nonlinear least-squares method, which fits the IF estimates to the predicted Doppler shift. Finally, the effectiveness of the proposed scheme is validated by using underwater acoustic data.

Forces and force-scaling in systems of adsorbing nanoparticles as measured using colloidal probe atomic force microscopy

Abstract Experimental force measurements (using colloidal probe atomic force microscopy, CP-AFM) were performed to measure the interactions between silica surfaces in the presence of highly charged nanoparticles. Experiments were done using 5 μm and 30 μm silica spheres as the colloidal probes in solutions of either sulfate latex (negatively charged) or zirconia (positively charged) nanoparticles. The forces were measured as a function of separation distance between the probe and a flat silica slide. The resulting force profiles indicated that the charged nanoparticles adsorbed onto the silica surfaces, producing strong, repulsive electrostatic forces. However, an unexpected result of the CP-AFM measurements was that when the repulsion arose primarily from the adsorbed nanoparticles and not the underlying silica, the repulsive forces between the probe particle and plate lacked the expected dependence on the radius of the probe as predicted by the Derjaguin approximation. Specifically, while the magnitude of the force was expected to scale linearly with the radius of the probe particle, the forces observed in nanoparticle suspensions were virtually identical for the 5 μm and 30 μm probes. By comparison, when the primary contributor to the repulsive force was the underlying silica, the correct scaling with radius was observed. Based on calculations of the shear rate in the gap, it was theorized that this result may arise from the shearing of adsorbed particles from the surfaces. Because the shear rate in the gap near the point of closest approach is independent of the size of the probe particle, this phenomenon could lead to small, similarly-sized patches of nanoparticles on the probe particles that effectively determine the electrostatic repulsion.

Analysis of moving source characteristics using polynomial chirplet transform.

The received Doppler signal of a stationary sensor, as emitted by a transiting acoustic source, is used to estimate source motion parameters, including speed, closest distance, rest frequency, and closest point of approach (CPA) time. First, the instantaneous frequency, amplitude, and CPA time are accurately estimated by the polynomial chirplet transform of the Doppler signal. Thereafter, the three other source motion parameters are obtained with a simplified amplitude-weighted nonlinear least squares method. The proposed scheme is successfully applied to the analysis of the characteristics of a moving truck.

Simulation study into the identification of nuclear materials in cargo containers using cosmic rays

Muon tomography represents a new type of imaging technique that can be used in detecting high-Z materials. Monte Carlo simulations for muon scattering in different types of target materials are presented. The dependence of the detector capability to identify high-Z targets on spatial resolution has been studied. Muon tracks are reconstructed using a basic point of closest approach (PoCA) algorithm. In this article we report the development of a secondary analysis algorithm that is applied to the reconstructed PoCA points. This algorithm efficiently ascertains clusters of voxels with high average scattering angles to identify `areas of interest' within the inspected volume. Using this approach the effect of other parameters, such as the distance between detectors and the number of detectors per set, on material identification is also presented. Finally, false positive and false negative rates for detecting shielded HEU in realistic scenarios with low-Z clutter are presented.

Binary Range-Rate Measurements and Homing Guidance

This paper considers the problem of planar homing guidance for a vehicle traveling to a beacon with unknown location and using a measurement of binary range-rate. The fundamental nature of binary range-rate is studied, and a guidance law requiring only this measurement is presented. Another guidance law utilizing both binary range-rate and heading angle is also presented, and stability is addressed. This guidance law consists of separated control and observation, and it does not require finding the closest points of approach for multiple headings. This guidance law's response to measurement corruption and observer gain is studied. Characteristics of good initial estimates are given, and an exploration method is presented that provides good initial estimates and leads to improved response. This approach provides a method of low-cost, autonomous homing guidance that utilizes a single, omnidirectional receiver to guide a vehicle to a single, omnidirectional transmitting beacon.

Beamspace time-frequency analysis for identification of underwater tone noise sources

The noise emitted by an underwater vehicle consists of several strong tones superimposed on a broad-band radiated noise component. Among them, the stable low-frequency tone noise induced by the reciprocating movements of the auxiliary machines in the underwater vehicle, carries characteristic information of the vehicle and is necessary for long-distance detection. Therefore, identification of the tone noise sources of an underwater vehicle is significant for noise reduction. On the basis of the joint information of space-time-frequency, beamspace time-frequency analysis (TFA) scheme is proposed for identification of low-frequency tone noise sources of underwater moving vehicle. First, the Doppler signals formed when the tone noise sources pass through the closest point of approach (CPA) are separated in time domain, by using superdirectivity beamforming of a small aperture circular array. The output signals can be approximated in linear form, i. e., LFM signal. After the LFM signals from the narrow beam are processed by two TFA methods of pseudo Wigner - Ville distribution and chirplet transform (CT), the time-frequency images of the noise signals are obtained. Then, the CPA time of each tone noise sources can be estimated by using peak search of the time-frequency images. At last, by converting the time coordinate to space coordinate and comparing with a reference source whose CPA time and position are known in advance, the positions of the low-frequency tone noise sources on the underwater vehicle are identified. The proposed scheme is different from the focused beamforming method, which scans the beam angle after eliminating the Doppler effect. Besides, due to no need of decorrelation usually used in the focused beamforming method, beamspace TFA scheme resolves the problem that array aperture is limited for identification of coherent noise sources of an underwater vehicle. The aperture of the used array can be reduced to meter-scale even when the frequencies of the tone noise are low. Although the array gain of superdirectivity beamforming decreases in nonisotropic noise field, the main lobe of the beam still keeps the same shape. Therefore, the performance of the proposed scheme is robust. Simulation analysis shows the following results: (1) Both the two beamspace TFA methods can precisely identify the underwater tone noise sources through a small aperture circular array, the radius of which is equal to 1.6 m, and the localization errors are less than 1 m when the signal-to-noise ratios are moderate; (2) The higher the frequencies of the tone noises are, the better the localization accuracy of beamspace TFA methods obtain; (3) The proposed scheme is less sensitive to the velocity of the underwater moving vehicle, and the localization results just have very small difference under various velocities; (4) The localization accuracy is related to distance, and decade meters is a reasonable choose for actual noise measurement; (5) Beamspace CT has better resolving accuracy when the information of measurement system is given, so the choice of the two beamspace TFA methods can be decided according to the actual measurement condition.

Operational iceberg drift forecasting in Northwest Greenland

An operational iceberg drift forecast model was developed and used in 2012 to support Shell Greenland A/S, which was leading a consortium of companies to conduct a scientific coring campaign offshore northwest Greenland. The nine-week program took place in August–October 2012 in Melville Bay where there is a very high spatial density of icebergs. In order to increase operational safety and efficiency, the iceberg drift model presented here was fully integrated within the ice management strategy and its decision protocols. The model was optimized to fulfill the operational requirements in terms of (1) the format and content of the input/output, (2) the calculation of the iceberg alert zones, and (3) the derivation of the associated Closest Point of Approach (CPA) and time to CPA (T-CPA). To achieve this, the iceberg drift forecast model used as near real-time input: in-situ measured metocean parameters, observed iceberg drift and size data, tidal currents, and weather forecasts. The model forecast the 24-hour iceberg drift trajectory including the CPA and T-CPA. It also derived the T-Time (e.g., Total Time, or the time necessary to lift the coring pipe to 50m below the seabed) alert zones based on the observed mean iceberg drift speed. The iceberg trajectory forecast was optimized by running a hindcast simulation of the observed iceberg trajectory. In this simulation, the air and water iceberg form drag coefficients were tuned to minimize the difference between the hindcast and the observed iceberg drift trajectories. The forecast iceberg track, beginning at the end of the observed iceberg record, was subsequently generated using the air and water form drag coefficients derived from the hindcast simulation. The model also incorporated a method to correct for the lack of robust near real-time in-situ ocean current measurements. During this campaign, the model was used to forecast the drift of 73 icebergs and exhibited good performances in locations with strong and persistent non-tidal currents. However, further offshore where the ocean currents were dominated by tidal and inertial forcings, the model exhibited poorer performance. Nevertheless, as shown for two icebergs in this paper whose drift was dominated by tidal and inertial oscillations, model performance could be significantly improved in the hindcast mode by using the vessel's wind data and the ocean current data retrieved from a mooring deployed in the area. This illustrated the importance of using adequate near real-time ocean current and wind measured and forecast input data.

Angle Statistics Reconstruction: a robust reconstruction algorithm for Muon Scattering Tomography

Muon Scattering Tomography (MST) is a technique for using the scattering of cosmic ray muons to probe the contents of enclosed volumes. As a muon passes through material it undergoes multiple Coulomb scattering, where the amount of scattering is dependent on the density and atomic number of the material as well as the path length. Hence, MST has been proposed as a means of imaging dense materials, for instance to detect special nuclear material in cargo containers. Algorithms are required to generate an accurate reconstruction of the material density inside the volume from the muon scattering information and some have already been proposed, most notably the Point of Closest Approach (PoCA) and Maximum Likelihood/Expectation Maximisation (MLEM) algorithms. However, whilst PoCA-based algorithms are easy to implement, they perform rather poorly in practice. Conversely, MLEM is a complicated algorithm to implement and computationally intensive and there is currently no published, fast and easily-implementable algorithm that performs well in practice. In this paper, we first provide a detailed analysis of the source of inaccuracy in PoCA-based algorithms. We then motivate an alternative method, based on ideas first laid out by Morris et al, presenting and fully specifying an algorithm that performs well against simulations of realistic scenarios. We argue this new algorithm should be adopted by developers of Muon Scattering Tomography as an alternative to PoCA.

Comparison Of AIS-Based Prediction Of The Distance At The CPA With Factual Separation Between Vessels

Abstract Since its deployment in 2004, the Automatic Identification System (AIS) has been considered a significant improvement of watchkeeping duties at sea. According to current regulations, AIS has not been recognised as an approved anticollision instrument yet. However, it would be difficult to rule out a possibility that AIS, being an essential part of the onboard SOLAS — compliant configuration, is unaidedly used for collision avoidance tasks. Recent research activities of DLR's Department of Nautical Systems have shown that AIS transmissions may contain a lot of incomplete data and the system does not have any dependable information on its data integrity. For that reason, the computation of the closest point of approach (CPA) and the time to the CPA (TCPA) are analysed based on AIS data involving multiple vessels, in order to compare the predictions with factual approaches between vessels and to evaluate the usability of AIS data, in its present form, for the appraisal of the traffic situation around each vessel.

SNARE and regulatory proteins induce local membrane protrusions to prime docked vesicles for fast calcium‐triggered fusion

Synaptic vesicles fuse with the plasma membrane in response to Ca(2+) influx, thereby releasing neurotransmitters into the synaptic cleft. The protein machinery that mediates this process, consisting of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and regulatory proteins, is well known, but the mechanisms by which these proteins prime synaptic membranes for fusion are debated. In this study, we applied large-scale, automated cryo-electron tomography to image an in vitro system that reconstitutes synaptic fusion. Our findings suggest that upon docking and priming of vesicles for fast Ca(2)(+)-triggered fusion, SNARE proteins act in concert with regulatory proteins to induce a local protrusion in the plasma membrane, directed towards the primed vesicle. The SNAREs and regulatory proteins thereby stabilize the membrane in a high-energy state from which the activation energy for fusion is profoundly reduced, allowing synchronous and instantaneous fusion upon release of the complexin clamp.

Target localization based on joint measurement of amplitude and frequency in a LOFAR field

To estimate the motion parameters of a moving target before its passing by the closest point of approach (CPA) point in a low frequency analyzing and recording (LOFAR) field, an error-free theoretical method based on the joint measurement of target radiated noise’s amplitude and frequency was presented. First, the error-free theoretical equations for target characteristic frequency, absolute velocity, the CPA, and amplitude of the radiation noise were derived by three equal interval measured values of the target amplitude and frequency. Then, the method to improve the calculation accuracy was given. Finally, the simulation and experiments were conducted in the air and showed the correctness of this method. By using one single piece of LOFAR, this method can calculate four target parameters and the relative error of each estimated parameter is less than 10%.

Particle filtering for obstacle tracking in UAS sense and avoid applications.

Obstacle detection and tracking is a key function for UAS sense and avoid applications. In fact, obstacles in the flight path must be detected and tracked in an accurate and timely manner in order to execute a collision avoidance maneuver in case of collision threat. The most important parameter for the assessment of a collision risk is the Distance at Closest Point of Approach, that is, the predicted minimum distance between own aircraft and intruder for assigned current position and speed. Since assessed methodologies can cause some loss of accuracy due to nonlinearities, advanced filtering methodologies, such as particle filters, can provide more accurate estimates of the target state in case of nonlinear problems, thus improving system performance in terms of collision risk estimation. The paper focuses on algorithm development and performance evaluation for an obstacle tracking system based on a particle filter. The particle filter algorithm was tested in off-line simulations based on data gathered during flight tests. In particular, radar-based tracking was considered in order to evaluate the impact of particle filtering in a single sensor framework. The analysis shows some accuracy improvements in the estimation of Distance at Closest Point of Approach, thus reducing the delay in collision detection.

Space base laser torque applied on LEO satellites of various geometries at satellite’s closest approach

In light of using laser power in space applications, the motivation of this paper is to use a space based solar pumped laser to produce a torque on LEO satellites of various shapes. It is assumed that there is a space station that fires laser beam toward the satellite so the beam spreading due to diffraction is considered to be the dominant effect on the laser beam propagation. The laser torque is calculated at the point of closest approach between the space station and some sun synchronous low Earth orbit cubesats. The numerical application shows that space based laser torque has a significant contribution on the LEO cubesats. It has a maximum value in the order of 10−8Nm which is comparable with the residual magnetic moment. However, it has a minimum value in the order 10−11Nm which is comparable with the aerodynamic and gravity gradient torque. Consequently, space based laser torque can be used as an active attitude control system.

Locality of surface interactions on colloidal probes

The Casimir and electromagnetic interactions between objects at short separations are strongly influenced by the local geometry near the point of closest approach. In this paper we demonstrate that the assumptions underlying common statistical analysis of roughness may not hold in experiments using micro-spheres as interacting objects. Based on an extensive experimental and numerical analysis of the surface topology of the widely used colloidal particle types 4310A and 4320A, we show that the actual variation in the local surface curvature may give rise to large uncertainties in the comparison of experimental data to theories.

Deep water towed array measurements at close range

During the North Pacific Acoustic Laboratory Philippine Sea 2009 experiment, towed array receptions were made from a towed source as the two ships transited from a separation of several Convergence Zones through a Closest Point of Approach at 3 km. A combination of narrowband tones and broadband pulses were transmitted covering the frequency band 79-535 Hz. The received energy arrives from two general paths-direct path and bottom bounce. Bearing-time records of the narrowband arrivals at times show a 35° spread in the angle of arrival of the bottom bounce energy. Doppler processing of the tones shows significant frequency spread of the bottom bounce energy. Two-dimensional modeling using measured bathymetry, a geoacoustic parameterization based upon the geological record, and measured sound-speed field was performed. Inclusion of the effects of seafloor roughness and surface waves shows that in-plane scattering from rough interfaces can explain much of the observed spread in the arrivals. Evidence of out-of-plane scattering does exist, however, at short ranges. The amount of out-of-plane scattering is best observed in the broadband impulse-beam response analysis, which in-plane surface roughness modeling cannot explain.

Interferometry of surfaces with well-defined topography in the surface force apparatus

We studied the multiple beam interferometry of surfaces with well-defined microscale features in the surface force apparatus (SFA). The structures investigated consist of hexagonal arrays of microscale cylindrical posts made out of the photoresist SU-8. The ability of the SFA to visualize the profile and topography of the interacting surfaces leads to the observation of discontinuities in primary fringes of equal chromatic order that are caused by the microscale structural features. The shift in wavelengths has been analyzed to extract the post height and compared to independent profilometry measurements. The analysis based on the shift in wavelength is shown to be viable only when the order of the fringe and the position of the discontinuity is precisely known and within the field of view. Analysis of the full profile of the interacting surface for two orthogonal 2-dimensional slices can be used to determine how the surface lattice is oriented within the point of closest approach. Finally, we discuss cases in which the structural features detrimentally affect the spatial resolution of the SFA.